1. Field of the Invention
The present invention relates to an apparatus used to obtain a satisfactory tomographic image by preventing fluctuation of a main magnetic field caused by oscillation of gradient coils in an open-type superconducting magnet for MRI (Magnetic Resonance Image Diagnostic Apparatus).
2. Description of Related Art
In an open-type superconducting magnet for MRI in the related art as is disclosed, for example, in JP-A-2002-52004 (paragraph [0045] and FIG. 6), a frame is provided to surround an overall superconducting magnet comprising a top superconducting magnet and a bottom superconducting magnet placed one on the other, and supporting members are provided to penetrate through holes made at the centers of the top superconducting magnet and the bottom superconducting magnet. One end of the supporting member is fixed to the frame and the other end is attached to the central portion of the gradient coil for fixing the gradient coil.
The open-type superconducting magnet for MRI comprises a pair of annular superconducting magnets provided one on the other, and generates a spherical magnetic field space measuring about 40 to 50 cm across having homogeneity of several ppm and a magnetic field strength of about 0.5 to 1 tesla almost at the center of a space between the superconducting magnets. Also, a pair of gradient coils that generates a pulsed gradient magnetic field in the highly homogeneous magnetic field space is attached to the end faces on the center side of the respective superconducting magnets. However, because the gradient coils are present between a pair of the annular superconducting magnets as described above and generate a pulsed magnetic field in a strong static magnetic field, they are susceptible to a large electromagnetic force and induce mechanical oscillation. The oscillation is transmitted to the superconducting magnets, which causes a distance between a pair of the superconducting magnets to vary and thereby gives rise to minute magnetic field oscillation. This results in a phenomenon that an MR image (Magnetic Resonance Image) is deteriorated.
The open-type superconducting magnet apparatus for MRI in the related art disclosed in the cited reference is configured as described above, so that mechanical oscillation induced in the gradient coils will not come into direct contact with a pair of the superconducting magnets. Oscillation transmitted to the superconducting magnets can be therefore reduced. However, because the need to provide a large frame to surround the superconducting magnets and the incapability of effectively fixing the outer peripheral portion of the gradient coils where the largest electromagnetic force is generated due to the structure for supporting the gradient coils only at the center, the open-type superconducting magnet apparatus for MRI in the related has a problem that oscillation and noise of the gradient coils are increased.